1. Field of the Invention
The present invention relates to a solid electrolytic capacitor that utilizes an electrically conductive polymer as a solid electrolyte.
2. Description of the Related Art
The development of digital devices in recent years has given rise to a demand for large-capacity capacitors which have excellent high-frequency characteristics; and studies have been made directed to the application of an electrically conductive polymer in this field.
Examples of known capacitors which have excellent high-frequency characteristics include thin-film, mica and ceramic capacitors. These conventional capacitors, however, involve the problem that, when they are designed to obtain an electrostatic capacity of 1 .mu.F or more, their size is increased, and the production cost is also raised considerably.
Electrolytic capacitors which are known to be large-capacity capacitors include two types, that is, the liquid electrolyte type in which a liquid electrolyte is impregnated, and the solid electrolyte type in which manganese dioxide is employed as a solid electrolyte. The former type of electrolytic capacitor which employs an electrolyte in a liquid state utilizes ion conduction, and there is therefore a remarkably increased resistance in the high-frequency region, whereby the impedance of the capacitor increases disadvantageously. The latter type of electrolytic capacitor employs as a solid electrolyte manganese dioxide and a charge transfer complex consisting of a combination of 7,7,8,8-tetracyanoquinodimethane (hereinafter referred to as TCNQ) and an electron donor.
The type of electrolytic capacitor which employs manganese dioxide as a solid electrolyte is obtained by thermally decomposing manganese nitrate since manganese dioxide is an insoluble solid substance. This type of electrolytic capacitor also has relatively high impedance in the high-frequency region, partly because manganese dioxide has a relatively high specific resistance, and partly because it is subjected to thermal decomposition several times to obtain manganese dioxide. Also a dielectric oxide coating provided on this type of electrolytic capacitor may be readily damaged, and the loss current of the capacitor thus increases to a remarkable extent.
The type of electrolytic capacitor which employs as a solid electrolyte a charge transfer complex consisting of a combination of TCNQ and an electron donor has already been proposed (see Japanese patent public disclosure No. 191414/1983, No. 17609/1983). However, this TCNQ charge transfer complex which has high electrical conductivity suffers from thermal stability and therefor involves a risk of the complex being decomposed during the capacitor manufacturing process to become an insulator.
A type of electrolytic capacitor which employs as a solid electrolyte a hetrocyclic polymer obtained by a electrochemical polymerization method has been proposed in recent years (see Japanese patent public disclosure No. 244017/1985, No. 2315/1986).
The electrolytic capacitor described above comprises an electrode having a dielectric oxide coating thereon and a solid electrolyte consisting of an electrically conductive hetrocyclic polymer which is formed by an electrochemical oxidation method. However, forming the electrically conductive hetrocyclic polymer on the dielectric oxide coating is impossible or, at least, difficult because the dielectric oxide coating is an insulator.